Piles are used to support structures where surface soil is weak by penetrating the soil to a depth where a competent load-bearing stratum is found. Helical (screw) piles represent a cost-effective alternative to conventional piles because of their speed and ease of installation and relatively low cost. They have an added advantage with regard to their efficiency and reliability for underpinning and repair. A helical pile typically is made of relatively small galvanized steel shafts sequentially joined together, with a lead section having helical plates. It is installed by applying torque to the shaft at the pile head, which causes the plates to screw into the soil with minimal disruption.
The main drawbacks of helical piles are poor resistance to buckling and lateral movement. Greater pile stability can be achieved by incorporating a portland-cement-based grout column around the pile shaft. See, for example, U.S. Pat. No. 6,264,402 to Vickars (incorporated by reference herein in its entirety), which discloses both cased and uncased grouted screw piles and methods for installing them. The grout column is formed by attaching a soil displacement disk to the pile shaft, which creates a void as the shaft descends into which flowable grout is poured or pumped. The grout column may be reinforced with lengths of steel rebar and/or polypropylene fibers. A strengthening casing or sleeve (steel or PVC pipe) can also be installed around the grout column. However, because the casing segments are rotated as the screw and the shaft advance through the soil, substantial torque and energy are required to overcome frictional forces generated by contact with the surrounding soil and damage to the casing material can result. Further, cased and grouted helical piles installed using current techniques and materials cannot necessarily be relied on to maintain their integrity during and after a cyclic axial and lateral loading event, such as an earthquake.